The present invention relates to a sample polishing apparatus and a sample polishing method for polishing a sample formed of a flat plate such as a semiconductor substrate, in particular, to configurations of fixed parts of a sample suck pad and a chuck base which hold the above sample.
As for a uniform flattening polishing method of an Si wafer, a semiconductor device wafer, or the like, a CMP (Chemical Mechanical Polishing) method is known which utilizes a precision polishing apparatus, a polishing slurry, a polishing pad, and the like and which is used for the flattening of an insulating film, for a capacitor formation, for an STI (Shallow Trench Isolation) and for the formation of a plug or a metal wire of Al, Cu, W, or the like.
A sample polishing apparatus (CMP apparatus) utilized in the CMP method comprises a platen of a large diameter where a polishing pad is stuck to the entirety of the upper surface thereof and a chuck base of a small diameter of which the lower surface faces the platen and which holds a sample as the polishing object such as a semiconductor substrate on that lower surface. The chuck base and the platen both have a thick disk form which are supported by respective supporting axes perpendicular to the respective faces opposite to each other in a coaxial manner so as to rotate around these supporting axes for the operation. In addition, a diamond electrodeposition ring is arranged on the same side of the platen as the chuck base so that the lower surface thereof faces the platen. A nozzle is arranged so as to face platen between the chuck base and the electrodeposition ring so that polishing slurry is supplied to the surface of the platen from the nozzle.
The chuck base is linked to a head which carries out the positioning in the horizontal direction and in the vertical direction on the platen. A chuck (sample suck pad) is stuck to the entirety of the lower surface of the chuck base, which faces the platen, via an adhesive layer so that a sample is fixed on the chuck. In addition, a retainer for preventing the sample from flying out and for preventing excessive polishing of the edges of the sample is provided on the lower surface of the chuck. The retainer is formed of a ceramic or of a specific resin such as Teflon (registered trademark) since chemical resistance and swelling resistance to the polishing slurry as well as resistance to abrasion from the polishing pad and from the polishing grains are required.
In the case that polishing is carried out by using such an apparatus, first, a sample is fixed on the chuck and the chuck base is positioned above a predetermined circle on top of the platen by means of the head so that both are rotated while being approached each other and then the surface to be polished(lower surface) of the sample is pressed on the polishing pad with a predetermined pressure so as to make contact with each other while grinding via the polishing slurry supplied from the nozzle. In addition, in order to stabilize the polishing rate, an in-situ dress of the polishing pad is carried out by the diamond electrodeposition ring for every predetermined number of samples being polished.
As described above, at the time of polishing process of an Si wafer and a semiconductor device wafer, polishing slurry is used. As for the polishing liquid of the polishing slurry, silica sol (pH≈10.5), ceria sol (pH≈6.5) or the like are used in the case of the flattening of an insulating film. Silica sol in general has a buffer solution as a base and pH thereof is fixed to alkaline by using potassium hydroxide, ammonia, or the like. The merits of this are cited as follows: the polishing rate is stabilized through the enhancement of dispersibility of the polishing grains and the polishing rate is increased through the hydration of the silicon oxide film. In addition, in the metal polishing using Al, Cu, W, or the like, alumina is generally used as the polishing grains and a method is used where the metal surface is mechanically peeled with the polishing grains while oxidizing with an oxidant such as hydrogen peroxide. The pH of the polishing slurry of this case is, in many cases, fixed in the range from 3.5 to the neutrality.
Since the alkaline or acid polishing slurry is, in many cases, utilized in this manner, the chemical resistance of the parts used in the CMP apparatus is important in enhancing the stability of the polishing apparatus. Particularly in the polishing process of a semiconductor wafer, the chuck structure for holding a sample is cited as a part which greatly influences the uniformity and the flattening of the wafer.
As for the chuck structure, a system of bonding a sample to a high precision flat plate by means of wax, a system of sucking and fixing a sample onto a high precision flat plate by means of vacuum suck and a template system wherein a sample is bonded onto a backing film which is a non woven fabric formed of a soft artificial leather are known. The bonding system by means of wax has a problem that lack of uniformity of the wax application greatly influences the polishing uniformity of the sample and the setting and releasing step through heating and cooling is necessary. And the template system by means of the backing film has a problem that aggregation grains are mixed in between the backing film and the wafer or a bubble is mixed in at the time of wafer suck so as to generate dimples and thereby the uniformity is deteriorated. In regard to these points, a vacuum suck system is adopted in many cases and the vacuum suck system has advantages that the number of polishing steps is smaller and the step management is simpler in comparison with other systems and the dimples on the wafer surface can be reduced.
In the case that the above described CMP apparatus is formed in accordance with the vacuum suck system, a chuck base made of ceramic where the lower surface is formed as a highly precise plane is utilized and pin holes of which the diameter is approximately 0.1 mm are provided at a few mm intervals over the entire surface of this chuck base. In addition, the chuck made of synthetic resin has been processed so as to have trenches on the suck surface toward a wafer so that the vacuum suck area is increased. In particular, for a sample which has a macroscopic waviness such as a semiconductor wafer, the vacuum suck and the surface standard polishing can be made effective by combining a chuck base which has the air permeability in the above manner and a chuck formed of a non woven fabric, rubber, resin, artificial leather, or the like which has the air permeability and the elasticity. Such a chuck structure is proposed by the present applicant in Japanese Patent Application Laid-Open No.8-181092(1996).
As described above, the chuck is stuck and fixed to the chuck base via an adhesive layer and the adhesive layer is formed of a commercially available acryl adhesive double coated tape or a resin based adhesive such as an epoxy resin or a light curing agent. In the case that the chuck is fixed to the chuck base by using an acryl adhesive double coated tape, however, the adhesive double coated tape is exposed to the acid and alkaline polishing slurry and the problem arises that the quality of the acryl resin on the outer periphery of the adhesive double coated tape is changed and this outer periphery becomes swollen due to moisture absorption so as to peel off from the chuck base. This is because the acryl acid ester (R1xe2x80x94COOxe2x80x94R2), which is the skeleton of the acryl adhesive double coated tape, undergoes hydrolysis in the acid or alkaline polishing slurry so that the adhesiveness is deteriorated.
In addition, in the case that the chuck is fixed to the chuck base by using a resin-based adhesive, there are problems that the elasticity is inferior to that of the adhesive double coated tape and therefore a pressure shock at the time of polishing movement cannot be buffered and in addition, the resin-based adhesive is swollen through the erosion by the polishing slurry and thereby the edge parts of the chuck is peeled off from the adhesive layer. In this manner, there is a problem that the edge parts of the chuck are peeled off and, thereby, the wafer cannot be stably fixed and the polishing uniformity of the wafer is deteriorated.
An object of the present invention is to provide a sample polishing apparatus and a sample polishing method wherein the quality of the adhesive layer is not changed through the exposure to a washing liquid, chemicals, or the like, so that the sample suck pad (chuck) can be prevented from being peeled off and the polishing uniformity of a sample can be increased.
In a sample polishing apparatus according to the first aspect of the present invention which sucks and holds a sample onto a sample suck pad (chuck) that is stuck to a chuck base via an adhesive layer so that the surface of the sample is polished by making contact with a polishing pad while grinding, the adhesive layer includes a rubber-based material. The sample suck pad is stuck and fixed onto the chuck base by means of the adhesive layer which includes a rubber-based adhesive material. The rubber-based adhesive material is excellent in the chemical resistance, the swelling resistance, the heat resistance, the adhesiveness and the peeling resistance in comparison with the acryl-based adhesive material and, therefore, even in the case that the bonding part between the sample suck pad and the chuck base is exposed to the acid or alkaline polishing slurry and the washing liquid, the peeling of an edge of the sample suck pad does not occur. As a result, the uniformity of the sample polishing increases. Here, the rubber-based adhesive materials obtained by mixing one or a plurality of tackifier(s) such as phenol-based resin, modified phenol resin, ketone resin, alkyd resin, rosin-based resin, coumarone resin, styrene-based resin, petroleum resin, or vinyl chloride-based resin into one or a plurality of adhesive rubber(s) such as chloroprene rubber, nitrile butadiene rubber, acryl rubber, styrene butadiene rubber, styrene isoprene styrene, styrene butadiene styrene, styrene ethylene butadiene styrene, butyl rubber, polyisobutylene rubber, natural rubber, or polyisoprene rubber, are effective.
A sample polishing apparatus according to the second aspect of the present invention is obtained by covering, with a sealing layer, the outer periphery of the adhesive layer, according to the first aspect, which includes a rubber-based adhesive material. Since the sample suck pad is stuck and fixed onto the chuck base by means of the adhesive layer which includes a rubber-based adhesive material and the outer periphery of this adhesive layer is additionally covered with a sealing layer, the uniformity of the sample polishing is further increased.
In a sample polishing apparatus according to the third aspect of the present invention, the adhesive layer according to the first or the second aspects is replaced with a rubber adhesive double coated tape. The rubber adhesive double coated tape is used in order to fix the sample suck pad onto the chuck base. The rubber adhesive double coated tape has a synthetic resin film as a base material where a rubber-based adhesive material is bonded through thermo compression on both sides thereof. By using an adhesive double coated tape which has a base material, the adhesive layer can withstand the pressure shock at the time of polishing movement and the adhesive layer can be easily renewed. As for the basic material of the adhesive double coated tape, polyester, polyethylene, polyvinyl chloride, non woven fabric or the like are used and in particular, a polyester based synthetic resin, of which the moisture absorption is small, is preferable.
A sample polishing apparatus according to the fourth aspect of the present invention is obtained by making the shearing adhesive strength of the adhesive layer is approximately 1 MPa or more in the apparatus according to the first, the second or the third aspects. The adhesive strength differs greatly according to the characteristics of the adhesive material and the property and the surface characteristics of the body to be stuck and moreover the utilization environment is moisture and the load is received repeatedly and, therefore, the setting of a quantitative value is difficult. In the case that the sample suck pad and the chuck base are made of synthetic resin or stainless steel, of which the surface a sample is stuck is finished with a mirror surface, however, the shearing stress generated by the polishing pressure in the range of from 4.9 kPa (50 gf/cm2) to 58.8 kPa (600 gf/cm2) is approximately 2.45 kPa (25 gf/cm2) to 29.4 kPa (300 gf/cm2). At this time, the shearing adhesive strength which can avoid the edge peeling caused by the impact at the initial time of the polishing movement and by the repeated stress is found to be 0.98 MPa (10 kgf/cm2) or more, that is to say, approximately 1 MPa or more after the rounding off. Here, in the case that an acryl adhesive double coated tape is used, the shearing adhesive strength thereof is approximately 392 kPa (4 kg/cm2) and this degree of shearing adhesive strength cannot withstand the impact at the initial time of polishing movement and the repeated stress so that the edge peeling occurs.
In a sample polishing apparatus according to the fifth aspect of the present invention which sucks and holds a sample onto the sample suck pad which is stuck to the chuck base via the adhesive layer and which polishes the surface of the sample by making contact with the polishing pad while grinding, the outer periphery of the adhesive layer is covered with a sealing layer. The outer periphery of the adhesive layer to which the sample absorption pad and the chuck base are stuck is covered with the sealing layer. The sealing layer which is formed by applying a sealing material with moisture resistance to the outer periphery of the adhesive layer can block the adhesive layer from the washing liquid or the polishing slurry so as to prevent the change in the quality, and the swelling, of the adhesive layer. As a result, the uniformity of the sample polishing is increased.
A sample polishing apparatus according to the sixth aspect of the present invention is obtained by forming the sealing layer in the apparatus according to the second or fifth aspect of an elastic sealing material. Since the outer periphery of the adhesive layer is covered with the elastic sealing material, the influence of the pressure shock which is subjected to the adhesive layer at the time of polishing movement is buffered. Here, the elastic sealing materials are silicon-based material, modified silicon-based material, polysulfide-based material, polyurethane-based material, acryl-based material, butyl rubber-based material, styrene butadiene rubber-based material, or the like, as one component materials and silicon-based material, modified silicon-based material, polysulfide-based material, polyurethane-based material, epoxy-based material, or the like, as two component materials. In regard to the adhesiveness to the sample suck pad and the chuck base as well as in regard to moisture resistance, a silicon-based material of one component, or two components, or polysulfide-based material of two components are particularly preferable.
In a sample polishing apparatus according to the seventh aspect of the present invention, the surface roughening is carried out on the surface of the chuck base bonded with the adhesive layer and/or on the surface of the sample suck pad bonded with the adhesive layer in the apparatus according to any of the first to the sixth aspects so that the average roughness is 0.5 xcexcm to 5.0 xcexcm. The wettability with the adhesive layer is increased by roughening the bonded surface so as to cause an anchor effect and at the same time scales are removed so as to increase the adhesiveness and, thereby, the edge peeling of the sample suck pad is further prevented. As for the roughness of the bonded surface, the average roughness is preferably 0.5 xcexcm to 5.0 xcexcm. In the case of the average roughness is smaller than 0.5 xcexcm, the wettability with the adhesive layer becomes insufficient in many cases so that there is the risk that the shearing adhesive strength will be reduced. In addition, in the case that the average roughness exceeds 5.0 xcexcm, air comes into recesses of the rough surface so as to cause gaps which reduce the wettability. In addition, the gaps become the cause of stress concentration so that there is the risk that the shearing adhesive strength will be reduced.
A sample polishing method according to the eighth aspect of the present invention, which sucks and holds a sample onto the sample suck pad which is stuck to the chuck base via the adhesive layer and which polishes the surface of the sample by making contact with the polishing pad while grinding, has the step of sucking a sample onto the sample suck pad which is fixed to the chuck base via the adhesive layer that is described in any of the first to the seventh aspects and the step of polishing the sample by making contact with the polishing pad while grinding. At the time of polishing the sample while supplying acid or alkaline polishing slurry, the quality of the rubber-based adhesive material is not changed by the polishing slurry and, therefore, edge peeling of the sample suck pad is not caused. In addition, the bonded part between the sample suck pad and the chuck base makes no contact with the polishing slurry by means of the sealing layer and, therefore, the adhesive layer does not swell so as not to cause the edge peeling of the sample suck pad. As a result, the uniformity of the sample polishing is increased.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.